I bought some under hood/firewall insulation, an aluminized side on a felt-like 1/2" pad. Don't remember where I got it...just searched e-bay!
I precut it to fit the inside of the dog=house and used a 3-M spray adhesive, got my wife to help install it, four hands are definitely better than two with the contact adhesive. I was a little worried that if it loosened up it could get sucked into the fan, so I put a layer of 1/2" hardware cloth on top of it and drilled some holes through the dog-house and ran small screws through the layers with fender washer and fibre lock nuts on the inside. Really works well
Also put Q-pads on top (inside) and the front (inside) of the fenders to cut the "tinny" sound of the metal, they also cut some sound transmission. Check your gasket on the dog-house to make sure it fits snugly.
I would say that the pinz is quieter that my old VW bus now,
you don't have to scream at each other...well you still have to talk a bit louder that in a new car.
I have read that some pinzers have had their dog-house sprayed with a rubber based bed liner and feel that that helps also.
That will be my next project.
Do an online search for insulating old volkswagens, there are many
great ideas for thermal and sound proofing.